The Java 2 Platform, Enterprise Edition (J2EE) specification defines one of the current standards for developing multi-tier enterprise applications. J2EE provides a component-based approach to the design, development, assembly, and deployment of enterprise applications, which both reduces the cost and enables fasten design and implementation. The J2EE platform gives the developer a multi-tiered distributed application model, the ability to reuse components, a unified security model, and flexible transaction control. Not only can they deliver innovative customer solutions to market faster than ever, but the resultant platform-independent J2EE component-based solutions are not tied to the products and application program interfaces (APIs) of any one vendor.
The J2EE specification defines the following kinds of components: application client components; Enterprise JavaBeans (EJB); servlets and Java Server Pages (JSP) (also called Web components); and applets. A multi-tiered distributed application model implies that the application logic is divided into components according to function, and different application components may make up a J2EE application on the same or different servers. Where an application component is actually installed depends on which tier in the multi-tiered J2EE environment the application component belongs. These tiers are depicted in FIG. 1. As shown therein an application server tier 104 is used to develop EJB containers and/or presentation containers such as servlets, JSP, and html pages 114. These in turn are used as an interface between a client tier 102, where the clients 108 and client applications are deployed, and a backend tier 106, used for hosting enterprise or legacy applications such Enterprise Resource Planning (ERP) systems.
Client tier—These can be browsers, Java-based programs, or other Web-enabled programming environments running within the client tier, both inside and outside of corporate firewalls.
Application Server tier—Normally this tier hosts a combination of presentation logic and business logic to support client requests. Presentation logic is supported via JSP pages and servlets that display HTML pages, while business logic is supported via Remote Method Invocation (RMI) objects and EJBs 112. EJBs rely upon the container environment for transactions, lifecycle and state management, resource pooling, security, etc., which together make up the run time environment in which the beans are executed.
Back-end tier—This is generally a combination of existing applications and data stores. It is also referred to as the Enterprise Information Systems (EIS) tier, since it may include such systems as Enterprise Resource Planning (ERP), mainframe transaction processing, database systems, and other legacy information systems.
Since the components of a J2EE application run separately, and often on different devices, there needs to be a way for client and application server tier code to look up and reference other code and resources. Client and application code can, for example, use the Java Naming and Directory Interface (JNDI) 116 to look up user-defined objects such as enterprise beans, and environment entries such as the location of the Java Database Connector (JDBC) DataSource objects, which in turn are used for looking up resources in backend tier, and message connections.
Application behavior such as security and transaction management can be configured at deployment time on Web and enterprise bean components. This deployment time feature decouples application logic from the configuration settings that might vary with the assembly. The J2EE security model lets a developer configure a Web or enterprise bean component so that system resources are accessed only by authorized users. For example, a Web component can be configured to prompt for a user name and password. An Enterprise Bean component can be configured so that only persons in specific groups can invoke certain kinds of its methods. Alternatively, a servlet component might be configured to have some of its methods accessible to everyone, and a few methods accessible to only certain privileged persons in an organization. The same servlet component can be configured for another environment to have all methods available to everyone, or all methods available to only a select few.
Some application servers, such as the WebLogic Server product from BEA Systems, Inc., San Jose, Calif., use an Access Control List (ACL) mechanism that allows for fine-grained control of the usage of components running on the server. Using an ACL, a developer can define at the Java Method level what can, or cannot, be executed by which user or group of users. This ACL mechanism covers anything that runs on the application server except for EJBs, which have their own access control mechanism defined in the EJ B specification. Security realms allow the administrator to import information from existing authorization or authentication systems into the ACL.
Java Servlets
A servlet is a program that extends the functionality of a Web server. A servlet receives a request from a client, dynamically generates the response (possibly querying databases to fulfill the request), and then sends the response containing an HTML or XML document to the client. Servlets are similar to CGI but are typically easier to write, since servlets use Java classes and streams. They execute faster because servlets are compiled to Java byte code and at run time the servlet instance is kept in memory, each client request spawning a new thread. Servlets make it easy to generate data to an HTTP response stream in a dynamic fashion. Each client request is performed as a new connection, so flow control does not come naturally between requests. To allow for this session management maintains the state of specific clients between requests. In some application servers, servlets make use of the HTTP session object to save their state between method requests. This object can be replicated in a clustered environment for fail over purposes.
Java Server Pages
JSP pages are a text-based, presentation-centric way to develop servlets. JSP pages offer all the benefits of servlets, and when combined with a JavaBeans class, provide an easy way to keep content and display logic separate. Both JSP pages and servlets are more desirable than Common Gateway Interface (CGI), because they are platform-independent, and use less overhead. JSP pages can be used with JavaBeans classes to define Web templates for building a Web site made up of pages with a similar look and feel. The JavaBeans class performs the data rendering, so the templates have no Java code. This means they can be maintained by an HTML editor. Simple Web-based application using a JSP page can be used to bind content to application logic using custom tags or scriptlets instead of a JavaBeans class. Custom tags are bundled into tag libraries that are imported into a JSP page. Scriptlets are small Java code segments embedded directly in the JSP page.
Java Messaging Services (JMS)
JMS is the J2EE mechanism used to support the exchange of messages between Java programs. This is how Java supports asynchronous communication, wherein the sender and receiver don't need to be aware of each other and thus can operate independently. JMS supports two messaging models:
Point to point—which is based on message queues. In this model message producer sends a message to a queue. A message consumer can attach itself to a queue to listen for messages. When a message arrives on the queue, the consumer takes it off the queue and responds to it. Messages can be sent to just one queue and will be used by just one consumer. Consumers have the option to filter messages to specify the exact message types they want.
Publish and subscribe—which allows producers to send messages to a topic and for all the registered consumers for that topic to retrieve those messages. In this case, many consumers can receive the same message.
One problem with current Servlet APIs is the completely synchronous programming model. After a request is dispatched to a particular thread the service( ) method of the appropriate servlet is called. When the service( ) method returns, the response is sent. This is a simple programming model which is suitable for many types of work, but is sub-optimal in the case that a asynchronous event must occur before the response can be sent, because the thread running the servlet must block until the event occurs.